<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script>
        let name="确定"
        //弹出用户名
        let myname=prompt('请输入你的用户名')
        alert(myname)
        //只声明变量，保存为undefined
        let sex

    </script>
</head>
<body>
<input type="button" value="按钮"></input>
</body>
</html>